Hydraulic exercise machines such as illustrated in U.S. Pat. Nos. 4,496,147; 4,480,832; 4,465,274; 4,363,481; 4,063,726; 3,702,188; 3,606,318; 3,530,766; 3,529,474; 3,128,094; and 2,079,594 have found favor because of the reliability inherent in hydraulic energy absorption systems that have so few moving parts and where there is no steel rubbing on steel. With respect to exercise stairs, three kinds of hydraulic stair systems have evolved. First, piston cylinder systems have been provided that offer hydraulic step height adjustment through a hydraulic bypass system described in U.S. Pat. No. 4,681,316. Other features of piston cylinder systems include hydraulic control for step height adjustment and long life under heavy use. Secondly, shock absorber based systems are used that are less expensive than piston cylinder systems, but do not offer the step height adjustment, rate control and the long life features of the piston cylinder systems. Thirdly, ram cylinder systems provide the cost advantages of the shock absorber based systems, and offer the rate control of the piston cylinder system, but no step height adjustment.
By way of definition, hydraulic piston cylinder systems utilize a plug, called a piston, that moves within the cylinder when a fluid under pressure is introduced into one end of the cylinder. The universal aspect of a piston is that it seals to the inside walls of a cylinder. As a result, the piston divides the cylinder into two distinct, isolated chambers. A piston rod is normally attached to the piston and extends through one of the two closed ends of the cylinder. In normal use, the piston applies a force through the rod to an object outside the cylinder as a result of fluid introduced to either the chamber above the piston or the chamber which is on the rod side of the piston. When used in an exercise device, the normal role of a hydraulic piston cylinder is reversed so that instead of turning energy into work, it acts like a pump which is activated by the user either pulling or pushing the rod, and converts his/her work into thermal energy.
Hydraulic ram cylinders are differentiated from piston cylinders by the absence of a piston. As a result, there is but one chamber in the cylinder. A portion of the rod or ram extends into the chamber and provides a surface for the fluid to push against. When fluid is introduced into this chamber, the ram is forced out of the cylinder. Rams can only push; they cannot pull. They also suffer some other deficiencies compared to piston cylinders depending upon application. However, they have two significant advantages. By having no pistons, they eliminate any problems associated with piston seal leakage; and they are inherently less expensive to manufacture than piston cylinders.
A shock absorber is merely a dashpot having a cylinder, with a piston and fluid being is compressed when the rod is pushed into or pulled from the shock absorber. In stair climbing apparatus, there is no hydraulic linkage between shock absorbers used for each step. Thus, hydraulic phasing and control is lacking in shock absorber systems.
Economics dictate which type of system is chosen for a particular application or market. The home market uses the shock absorber and ram cylinder systems, giving up the step height adjustment and life features of piston systems in exchange for a significant reduction in cost. On the other hand, the health club and other institutional markets utilize piston systems, although at a high price, for long life and the convenience of hydraulic speed and step height control.
It will be noted that the great discrimination inherent in hydraulic speed control is important in setting the precise prescription level of exercise intensity. However, speed is only half of the intensity equation. Work is a function of force and distance. The level of exercise intensity in stair climbing is determined by stepping rate and step height. Thus, one must have step height control as well as rate control for intensity control. Step height adjustment is not presently available for ram cylinder systems.
In addition to importance in measuring and setting exercise intensity, step height is a major factor in the comfortable use of a stair exercise machine. Stair climbing step height is as precise as a runners stride. A 1/4 inch adjustment can turn an uncomfortable workout into a comfortable one, especially at the higher intensity levels. In piston systems hydraulic bypass adjustment provides for easy and precise step height adjustment to accommodate the needs of various height users, and users wishing to limit knee or ankle excursion. Thus, for stair climbing apparatus, step height control has heretofore only been available in piston cylinder systems via the aforementioned bypass technology.
Additionally, step height adjustment in the paired piston cylinder stair climbing hydraulic systems is a by-product of the reclamation of fluid lost Past the piston as a result of piston seal leakage. When the bypass is opened to reclaim lost fluid, the step height can be set to any portion of the maximum stroke, and locked in that position by closing the bypass. This bypass adjustment feature cannot be applied to paired ram cylinder stair climbing hydraulic systems because the rams do not seal the cylinders into separate chambers. Consequently, the ram cylinder stair climbers brought to market do not offer step height adjustment.
Although quick step height adjustment is of prime importance for ease of exercise, step height adjustment permits compact storage for units designed for home use. One such home use machine is the rotary arm stair which is designed to be stored under a bed or in a crowded closet which is important in space limited homes and apartments. With rapid step height adjustment, the arms can be rotated down so the entire apparatus can be slipped under a bed.
By way of further background, with respect to fluid loss, all hydraulic cylinders, including shock absorbers, must lubricate their rod seal in order to maintain its life. A very small amount of fluid is carried through the seal on the rod and is left outside of the cylinder on each stroke. This fluid loss is called carry out.
Fluid loss is proportional to the number of strokes. This becomes important considering that 5 million strokes per year for machines in health clubs is common; as is 1/2 million strokes per year for home machines.
The conventional approach to accommodating carry out in the shock absorber and ram cylinder exercise systems has been to make the initial step so high that the loss in step height caused by carry out is not objectionable. However, high initial step height is uncomfortable. Also, the uncomfortably high step height often degrades through carry out to an unusable low step height. Thus, the effective life of the machine is severely limited.
As an additional problem, all exercise energy absorption systems transfer the user's work into heating the atmosphere. To accomplish this energy transfer, the temperature of the hydraulic system must rise above ambient, and as a consequence, the fluid in the system expands. The expansion adds to the step height and is proportional to the amount of user's energy being dissipated and the coefficient of expansion of the fluid. In the current shock absorber and ram cylinder machines, this thermal expansion can make an uncomfortably high step height even higher and more uncomfortable.